Compositions and methods for planarizing or polishing the surface of a substrate, especially for chemical-mechanical polishing (CMP), are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. The polishing composition is typically used in conjunction with a polishing pad (e.g., polishing cloth or disk). Alternatively, the abrasive material may be incorporated into the polishing pad.
Polishing compositions for silicon-based inter-metal dielectric layers have been particularly well developed in the semiconductor industry, and the chemical and mechanical nature of polishing and wear of the silicon-based dielectrics is reasonably well understood. One problem with the silicon-based dielectric materials, however, is that their dielectric constant is relatively high, being approximately 3.9 or higher, depending on factors such as residual moisture content. As a result, the capacitance between the conductive layers is also relatively high, which in turn limits the speed (frequency) at which a circuit can operate. Strategies being developed to reduce the capacitance include (1) incorporating metals with lower resistivity values (e.g., copper), and (2) providing electrical isolation with insulating materials having lower dielectric constants relative to silicon dioxide.
One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as titanium or tantalum and/or a diffusion barrier layer such as titanium nitride or tantalum nitride. The adhesion-promoting layer and/or the diffusion barrier layer are then over-coated with a copper layer. Chemical-mechanical polishing is employed to reduce the thickness of the copper over-layer, as well as the thickness of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
Previously, it was believed that the removal rate of the copper and the adhesion-promoting layer and/or the diffusion barrier layer must both greatly exceed the removal rate of silicon dioxide so that polishing effectively stops when elevated portions of the silicon dioxide are exposed. The ratio of the removal rate of copper to the removal rate of silicon dioxide base is called “selectivity.” A minimum selectivity of about 50 was desired for such chemical-mechanical polishing. However, when high selectivity copper slurries are used, the copper layers are easily over-polished creating a depression or “dishing” effect in the copper vias and trenches. Furthermore, a certain amount of overpolish during copper CMP is required to clear off all surface metal residue between metal patterns and to ensure electrical isolation between neighboring circuits. During overpolish, multiple materials are polished simultaneously at different polishing rates. Overpolishing, therefore, can result in copper dishing and silicon dioxide erosion, which are highly pattern dependent. Copper dishing results from the selectivity of copper polishing systems for copper, whereas erosion refers to a topography difference between regions with no metal pattern or very low metal pattern and dense arrays of copper trenches or vias. The industry standard for erosion is typically less than 500 Angstroms (Å).
A number of systems for chemical-mechanical polishing of copper have been disclosed. Kumar et al., “Chemical-Mechanical Polishing of Copper in Glycerol Based Slurries” (Materials Research Society Symposium Proceedings, 1996) discloses a slurry that contains glycerol and abrasive alumina particles. Gutmann et al., “Chemical-Mechanical Polishing of Copper with Oxide and Polymer Interlevel Dielectrics” (Thin Solid Films, 1995), discloses slurries based on either ammonium hydroxide or nitric acid that may contain benzotriazole (BTA) as an inhibitor of copper dissolution. Luo et al., “Stabilization of Alumina Slurry for Chemical-Mechanical Polishing of Copper” (Langmuir, 1996), discloses alumina-ferric nitrate slurries that contain polymeric surfactants and BTA. Carpio et al., “Initial Study on Copper CMP Slurry Chemistries” (Thin Solid Films, 1995), discloses slurries that contain either alumina or silica particles, nitric acid or ammonium hydroxide, with hydrogen peroxide or potassium permanganate as an oxidizer. While present day chemical-mechanical polishing systems are capable of removing a copper over-layer from a silicon dioxide substrate, the systems do not entirely satisfy the rigorous demands of the semiconductor industry. These requirements can be summarized as follows. First, there is a need for high removal rates of copper to satisfy throughput demands. Secondly, there must be excellent topography uniformity across the substrate. Finally, the CMP method must minimize local dishing and erosion effects to satisfy ever increasing lithographic demands.
To this end, copper CMP compositions have been devised which include inhibitors of copper overpolishing. Typically, such inhibitors comprise nitrogen-containing compounds, for example, amines and small molecular weight nitrogen-containing heterocyclic compounds such as benzotriazole, 1,2,3-triazole, and 1,2,4-triazole. For example, U.S. Pat. No. 6,585,568 describes a CMP polishing slurry for polishing a copper-based metal film formed on an insulating film, comprising a polishing material, an oxidizing agent, and water as well as a benzotriazole compound and a triazole compound, wherein the mixing ratio of the triazole compound to the benzotriazole compound is 5 to 70. U.S. Pat. No. 6,375,693 discloses a slurry for polishing a tantalum-based barrier layer for copper-based metallurgy, consisting of hydrogen peroxide for oxidizing copper, a copper oxidation inhibitor, an additive that regulates complexing between copper and the oxidation inhibitor, and colloidal silica, wherein the oxidation inhibitor is selected from the group consisting of 1-H benzotriazole, 1-hydroxybenzotriazole, 1-methylbenzotriazole, 5-methylbenzotriazole, benzimidazole, 2-methylbenzimidazole, and 5-chlorobenzotriazole. The purpose of the copper oxidation inhibitor is to reduce etching of copper within trenches by hydrogen peroxide after the copper layers have been planarized.
However, despite the improvements achieved in the reduction of dishing and erosion in the CMP of copper-containing substrates with the use of small molecular weight heterocyclic copper inhibitors, it is well known that excessive amounts of inhibitors, such as benzotriazole, result in a reduction of the polishing rate for copper, forcing a tradeoff between erosion inhibition and attainment of practical polishing rates. Furthermore, the use of copper corrosion inhibiting compounds, such as triazole compounds, in CMP compositions has been found to increase the incidence of defects, such as precipitates and copper stains. Thus, there remains a need in the art for improved polishing systems and methods for the chemical-mechanical planarization of substrates comprising copper layers.
The invention provides such a system and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.